POWERMASTER EXPERTS

PaulRV6

Active Member
Joined
May 25, 2001
I have been diagnosing this problem for a while now and need to get a few more questions answered. I replaced the ball and hooked up the test fixture and here are the results.
The pump settles out at 640 psig
Turns on at 480 psig
The switch closes at 350 psig
The accumulator pre-charge is right at 300 psig
All this seems to check out ok but the pedal can be pushed all the way down with enough foot pressure. I don't believe it should be this way. Please confirm?
The other thing is that the pump turns on every other time I push on the pedal. Is this normal?
There are no leaks in the system and I bled the brakes so there is no air in the system.
I'm thinking that the master cylinder could be bad and therefore leaking by the seals causing the pedal to go down so far. I am not aware of any method to check the MC for this. Is there any? What are your thoughts of this? Thanks kindly.

paul

PS I am not interested in changing the brake system as I am set on getting this to work. Thanks
 
The brake pedal should not go all the way to the floor.
On my GN, if I step on the brake pedal with no power assist, the pedal goes down a bit and stops.
If I hold my foot on it, it does not go down further. If it did go down further, that could mean bad seals
in the master cylinder. When I turn my key on and the pump runs and yes, my pedal goes down a bit more.
but not to the floor. That is a result of things flexing expanding in the brake system with the added pressure of the assist.
All things have some elasticity, brake drums flex, piston seals compress etc.
It is normal for the pump to run after a one or two pumps.
I'm not sure without looking it up but I think the ball is normally charged higher then 300 psi.
How are the flexible brake lines on the car? I've heard they can swell under pressure when they get old.
That could cause a spongy brake pedal.
I'm curious, have you tried to drive the car a little and step on the brake?
These brakes when working well are sensitive.
My GN and T-type will chirp the tires when I stab on the brakes at even a slow roll.
My other cars without power masters do not seem as sensitive.
Are you anywhere near another turbo Buick owner to compare?
 
This should be obvious, but need to say: Inspect wheel cyls, hoses and calipers for external leaks.

Air in the base brake system would cause a low or spongy pedal. You bled the system, so this shouldn't be an issue. If the pedal falls away with constant pressure, you may have leaking piston seals in the MC section for the PM unit.

There are no internal parts available to repair the seals. GM released a kit many moons ago, but parts are non-existent today.
 
I have been diagnosing this problem for a while now and need to get a few more questions answered. I replaced the ball and hooked up the test fixture and here are the results.
The pump settles out at 640 psig
Turns on at 480 psig
The switch closes at 350 psig
The accumulator pre-charge is right at 300 psig
All this seems to check out ok but the pedal can be pushed all the way down with enough foot pressure. I don't believe it should be this way. Please confirm?
The other thing is that the pump turns on every other time I push on the pedal. Is this normal?
There are no leaks in the system and I bled the brakes so there is no air in the system.
I'm thinking that the master cylinder could be bad and therefore leaking by the seals causing the pedal to go down so far. I am not aware of any method to check the MC for this. Is there any? What are your thoughts of this? Thanks kindly.

paul

PS I am not interested in changing the brake system as I am set on getting this to work. Thanks
Hi Paul,
What type of test fixture you used to check the results?
 
I'm getting back into my brakes. When I turn the ign on the PM pump runs and sucks down the one side of the reservoir. After a few minutes the reservoir begins to fill back up without doing anything. Is this suppose to be that way? It just seems like the pump would be turning on much too often. Like I stated earlier, the accumulator and switch are good by the pressure readings and the pump seems to be working good. What am I missing. Thanks

paul
 
No. It's likely the check valve is leaking. The check valve can be removed in the car. Discharge the system, remove the pressure switch, and line from the power master pressure side. Then use a shot of air to remove the check valve assembly. Be sure to cover the fender when removing the check valve to avoid spraying the brake fluid over it. If the face of the check valve is not all marred up clean it off and re-assemble. If it's a little nasty you can try sanding it in a hard flat surface with some very fine sand paper.


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Bison, I'm looking at the gm manual on the pm section and the only item I can find for a check valve is items 36, 37, 38 called a valve seat and seal poppet. Is this what you are referring to? Do I have to drain the reservoir before removing? Thanks for the help.
Paul
 
John Norton and Richard Clark just wrote a very detailed booklet on how to test and check the Powermaster using simple tools etc....very detailed manual with photos. We have them in stock. I suggest you purchase this manual. From what you are describing sounds like dirt is causing issues in one of the check valves. Also what would help is replace the frotn rubber style brake hoses with the new stainless ones they cut down pedal travel as the rubber hoses tend to swell being 25 plus years old.

kirbanperformance.com

denniskirban@yahoo.com

I am not an expert but do have the parts to help.
 
There are two ports/holes in the bottom of the passenger side reservoir. One toward the front and one toward the back. Depress the brake pedal with the key on to force the pump to run. When the pump stops place a piece of vacuum hose over each port. If either one is leaking,it will flow out of the top of the hose as long as you have a good seal over the port. Which one spills over?
 
In addition to the check valve, there is the valve which controls the assist.
As you step on the pedal, this valve opens and lets pressure help apply the brakes.
This valve wears. It forms a groove. When the mating surface doesn't seat in the groove,
it will bleed off pressure. The cure is to dis-assemble this part of the master cylinder and
spin the valve on a lathe and polish out the groove with 600 paper. Fortunately,
it is one of the first things you find when you dis-assemble the master cylinder.
If the PM unit sat for awhile, it could have rusted and the pits are letting pressure bleed off as well.
There is not cure for badly rusted valve components.
 
Yes ck valve is 36, 37, 38 in diagram. It would be a great benefit to this community if someone could mfg this valve.
 
It hasn't been determined that the OP's check valve is bad. There are two ports at the bottom of the booster side reservoir. We first need to determine which port the fluid is returning to the reservoir through. The front port is the check valve. The rear port is the port that fluid flows through when the booster portion is leaking. The most common cause of this leak is eroded O-rings on the boost piston.
 
Ttype6, it is the forward port that is leaking out after pump stops running. I'm assuming it shouldn't fill bac up? Is that my problem?
What do I do next. Thanks
Paul
 
Ttype6, it is the forward port that is leaking out after pump stops running. I'm assuming it shouldn't fill bac up? Is that my problem?
What do I do next. Thanks
Paul
This is the port for the check valve. Do what Bison suggested in post #7.
 
Is it difficult to remove the check valve while the mc is in place? Would I be better off removing the mc to do this? Thanks allot.
Paul
 
I removed the accumulator and the steel line going into the MC. Where is the check valve? It doesn't seem like it could be inside the boss on the side since the steel line has seal against a flared seat inside the hole. Where do I blow the air into and where does the check valve fly out of? Thanks
 
I removed the accumulator and the steel line going into the MC. Where is the check valve? It doesn't seem like it could be inside the boss on the side since the steel line has seal against a flared seat inside the hole. Where do I blow the air into and where does the check valve fly out of? Thanks
 
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